1. Field of the Invention
The present invention relates in general to measuring a liquid level in a vessel, and in particular, to an acoustic method and apparatus for measuring water level in a closed vessel such as a pressure vessel.
2. Description of the Related Art
Liquid levels in vessels have been determined from sight glasses in which the liquid is observed visually. Levels are also determined by measuring the difference in pressure between the top and bottom liquid/gas layers of the vessel. These methods ordinarily require at least two penetrations into a vessel wall. Ideally, a water level detection system for a vessel subjected to high temperatures and pressures should have no vessel penetrations and there should be no active components inside the vessel due to the potential failure of these mechanisms.
It is known to use ultrasound for measuring the depth of a liquid or a dry solid such as grain. Sound travels through the liquid, or through the air space above the liquid or solid. In these cases, this normally requires a transducer situated inside the vessel or container.
Other methods have been proposed, but they all use at least one penetration of the wall and subject the sensors to high temperatures and pressures.
There exists an electronic level fluid measurement system that uses ultrasonic sensors epoxied externally to a bottom of a tank. This system measures liquid level up through the tank bottom, "bouncing" signals off of the liquid/air barrier and then computing volume from signal rebound time. It is available from Sandeno Technical, 20016 Cedar Valley Road, Lynnwood, WA 98036.
A disadvantage with this system is that heat affects ultrasonic sensors. If there are any internal structures or tilting, it can prevent proper operation.
Thus, there is a need for a liquid level detection system suitable for monitoring liquid levels of vessels subjected to high pressures and temperatures. The liquid level detection system should not penetrate any vessel walls and the active components should be remotely located from the vessel.